- He was around during the Great Sith War and his students either didn't turn on him or they didn't manage to kill him. He also probably predicted in the final attack on Yavin 4. Of course, we can assume Yoda would be able to do the same if he was put it such a situation.
Hu what? We never even see Vandar before Kotor he was around that much is certain, but what he did why and how we don't know..
- During the Jedi Civil War, Vandar presumably predicted Malak's attack and was among the Enclave survivors. As far as Yoda, without Sidious around, Yoda is capable of predicting a great many things.
Perhaps or perhaps he was just hanging out on Coruscant with the rest of the High Council, or maybe he was eating some apple pie with the girl next door...
- However, Vandar couldn't counter Bastila's battle meditation that well. Yoda probably could, since he is also advanced in the art of battle meditation and presumably knows how to counter it.
Nobody could have countered that, she was a Battle Meditation goddes, not even amongst the masters was there one as good as her. It was a special talent she had that made her very important to both the Jedi and the Sith during the war, not being able to counter that does not speak against him. And its very unlikely that yoda would have been able to counter it.
for the rest however, Yoda could have very well called for a meeting like that, or perhaps he would not have. Vandar possibly was or wasn't on the high council during the time of Exar Kun, and we don't know if he fought in any battles there. He did train Revan and Revan turned against him so saying that he never had a student turn against him is also wrong.
All in all we still have no idea on the power of Vandar.